Silicone resins that contain 1.5 oxygen atoms per silicon atom are generically known as polysilsesquioxanes. Polysilsesquioxanes are highly heat resistant and exhibit good electrical insulation properties and flame retardancy, and this good property spectrum has resulted in their use as resist materials and interlayer dielectric films in semiconductor fabrication (for example, see, "Shirikoon Handobukku" (English title: Silicone Handbook), edited by Kunio Itoh, published by Nikkan Kogyo Shinbunsha (1990)).
Methods are known for the synthesis of polymethylsilsesquioxanes. For example, polymethylsilsesquioxane can be synthesized by dissolving methyltrichlorosilane in the presence of an amine in a single solvent or mixture of solvents selected from ketones and ethers, adding water to this system dropwise to effect hydrolysis, and then heating to effect condensation such as is disclosed in Japanese Patent Publication Nos. 60-17214 (17,214/1985) and 1-43773 (43,773/1989), and in U.S. Pat. No. 4,399,266. Another synthesis example is taught in EP 0 406 911 and Japanese Patent Publication No. 62-16212 (16,212/1987) which teach the dissolution of a trifunctional methylsilane in organic solvent, then hydrolysis by the dropwise addition of water to this solution at a temperature from -20.degree. C. to -50.degree. C. under an inert gas pressure of 1,000 to 3,000 Pa, and thereafter condensation by heating. Yet another synthesis example is disclosed in Japanese Patent Application Laid Open No. 3-20331 (20,331/1991) which teaches the reaction in organic solvent of methyltriacetoxysilane with an equivalent amount of alcohol and/or water to synthesize the alkoxyacetoxysilane, polycondensation of the alkoxyacetoxysilane in organic solvent in the presence of sodium bicarbonate to give a prepolymer, and condensation of this prepolymer by heating in the presence of at least 1 catalyst selected from the alkali metal hydroxides, alkaline-earth metal hydroxides, alkali metal fluorides, alkaline-earth metal fluorides, and triethylamine. Still another synthesis example is found in Japanese Patent Application Laid Open No. 3-227321 (227,321/1991) which teaches the dissolution of alkali metal carboxylate and lower alcohol in a mixed liquid system that forms two phases (water and hydrocarbon solvent), the dropwise addition of methyltrihalosilane into this system to effect hydrolysis, and condensation by heating.
The polymethylsilsesquioxanes afforded by these methods are hard but brittle. Japanese Patent Publication No. 1-43773 further discloses that regulating the fraction with molecular weight .ltoreq.20,000 (molecular weight as determined by gel permeation chromatography (GPC) calibrated with polystyrene standards) to 15 to 30 weight % of the polymethylsilsesquioxane. However, even this does no more than enable the preparation of coatings with thicknesses of about 1.8 to 2.0 .mu.m. Similarly, the technology in EP 0 406 911 can only provide coatings with maximum thicknesses of 3 to 3.5 .mu.m without cracking. At larger film thicknesses cracking occurs, and of course the flexibility that would permit the fabrication of an independent or stand-alone film is absent.
We have already discovered and have taught in Japanese Patent Application Nos. 7-208087 (208,087/1995) and 7-208143 (208,143/1995) that a coating that combines flexibility with high thermal stability is provided by the cure of a polymethylsilsesquioxane having a molecular weight and hydroxyl content in specific ranges and preferably prepared by a special method.
On the subject of the silylation of the residual silanol in polysilsesquioxane, a synthetic method for this is disclosed in, for example, J. Am. Chem. Soc., 1990, Volume 112, pages 1931 to 1936. Japanese Patent Application Laid Open No. 61-221232 (221,232/1986) teaches a method for the preparation of silylated polysilsesquioxane comprising the use of silylating agent to terminate the reaction in the polysilsesquioxane synthesis methods of EP 0 406 911 and Japanese Patent Publication No. 62-16212. Japanese Patent Application Laid Open Nos. 6-279586 (279,586/1994), 6-287307 (287,307/1994), and 7-70321 (70,321/1995) teach that stabilization can be achieved and gelation can be avoided by trimethylsilylation of the silanol in polysilsesquioxane in which methyl constitutes 50 to 99.9 mole % of the pendant organic groups and crosslinking-reactive groups are present in the remaining organic groups. However, even without silylation the polymethylsilsesquioxane disclosed by us in Japanese Patent Application Nos. 7-208087 and 7-208143 does not gel during its preparation and can be stably stored at room temperature. Japanese Patent Application Laid Open No. 5-125187 (125,187/1993) teaches that an increased storage stability can be obtained by trialkylsilylation of the silanol in polysilsesquioxane having a number average molecular weight (Mn) .gtoreq.100,000 and methyl as 50 to 100 mole % of its pendant organic groups. Japanese Patent Publication No. 62-16212 also teaches that silylation of the silanol in polymethylsilsesquioxane improves stability.
Organic groups in polyorganosilsesquioxanes, and polyorganosilsesquioxanes functionalized with various crosslinking-reactive groups are described in, for example, Chem. Rev., 1995, Volume 95, pages 1409 to 1430.
As is already well known, the release of various tacky or sticky substances can be facilitated and substrate blocking can be prevented by coating a polyorganosiloxane-based release coating composition on a substrate such as paper or plastic film, and curing the composition by a condensation or hydrosilylation reaction.
A problem generally encountered with the polyorganosiloxane-based release coating compositions has been the extremely low release resistance of the cured release coatings provided by the cure of these compositions. This has led to the addition of controlled release additives to these compositions in order to adjust the release resistance. The heretofore proposed controlled release additives are exemplified by MQ silicone resins comprising R.sup.7.sub.3 SiO.sub.1/2 units wherein R.sup.7 is a monovalent hydrocarbon group containing no more than 2 carbons and SiO.sub.4/2 units as is disclosed in Japanese Patent Publication No. 49-27033 (27,033/1974) and Japanese Patent Application Laid Open No. 59-84953 (84,953/1984)) and MQ silicone resins comprising R.sup.7.sub.3 SiO.sub.1/2 units, (CH.sub.2 .dbd.CH)R.sup.7.sub.2 SiO.sub.1/2 units, and SiO.sub.4/2 units as is disclosed in Japanese Patent Publication No. 53-29678 (29,678/1978). However, even with the use of controlled release additives as described above, the release resistance of the cured coating can undergo a substantial decline due to release paper aging or release liner aging.